Unlike previous video coding standards, H.261, H.263, MPEG2 or MPEG4, a succession of macroblocks or slice of macroblocks, can be made up of non-consecutive macroblocks in the image, depending on the television type image scanning direction. This possibility is called flexible macroblock ordering (FMO). This tool is described, for example, in the contribution by S Wenger and M Horowitz, JVT-C089 ISO/MPEG & ITU, May 2002.
In the context of video image transmission over multi-vendor networks, in which normally the high transmission error rate is characterized by the loss of packets, there is a need to protect the packets of compressed data using, among other things, error correcting codes such as the Reed-Solomon codes or the forward error correction (FEC) continuous checking codes. These error correction algorithms, based on redundancy, and therefore expensive in terms of coding cost, do not, however, prioritize areas of images deemed important for coding.